Production of coated sand



United States Patent @fiice 2,888,418 Patented May 26, 1959 PRODUCTIONOF COATED SAND John Albanese, Maywood, and Chester W. Fitko, Chicago,Ill.

No Drawing. Application October 22, 1953 Serial No. 387,786

4 Claims. Cl; 260-28) This invention relates to improvements in theproduction of sand precoated with resin and particularly adapted for usein the shell molding process.

More particularly, the present invention relates to improvements in themethod of producing storage-stable, substantially free-flowing sandparticles whereby each sand grain is provided with a solid, nontackyenveloping coating of potentially thermosetting phenolic resincomposition and an outermost coating of solid, nontacky, unctuousmaterial, and to the novel properties, utility and advantages of theresulting product.

The shell molding process for the production of sand mold sections forthe casting of metals as known in the prior art has in general comprisedthe employment of an essentially dry mixture of sand and powdered resinand when required powdered resin curing and accelerating agents, and thedepositing of such mixture against the face of a heated metal patterncoated with a lubricating or release agent such as a silicone resin.This is generally accomplished by placing the aforesaid dry mixture inthe bottom of a pivotally supported or other container with the topclosed by the metallic pattern. After the pattern is preheated to about400-500 F., the container is inverted so that the sand-resin mixture isthrown against it, and in less than a minute the resin melts and fusesand forms a unitary crust of from about /8 to about /2 inch inthickness. Excess sand-resin mixture, which has not been reached by heatin an amount to fuse the resin, is removed for subsequent use, and thepreliminary shell or crust on the pattern is subjected to further heat,as in an oven, to cure the resin to the thermoset or infusiblecondition. It is then separated from the pattern in the form of a rigidshell mold section ready for use for the casting of metals.

As distinguished from the employment of dry sandresin mixtures in theaforesaid process, the copending applications of Fitko and Horn, SerialNos. 297,396, filed July 5, 1952, now Patent No. 2,706,188, and 346,534,filed April 2, 1953, teach the production of storage-stable,free-flowing sand grains, each provided with a solid, nontacky coatingof potentially thermosetting phenolic resin for use in the aforesaidshell molding process in lieu of the dry sand-resin mixture' Thiseliminates dust and explosion hazards of a powdered mix, avoidsseparation and Stratification of the components, permits employment ofconventional foundry sand handling equipment, permits the production ofshell molds of more uniform character and of improved physicalproperties, and saving in the amount of resin required.

The processes of the aforementioned Fitko and Horn copendingapplications for coating sand with resin comprise in general the placingof foundry sand into an intensive mixing vessel, together with fromabout 2 to 4% by weight of the sand of acid-catalyzed, fusiblephemol-formaldehyde resin, i.e., novolak. These are mixed together touniformly distribute the resin over the individual sand grains, with theaid of heat adequate to maintain the resin, whether it be initiallynormally solid or normally liquid, in liquid condition. While the resinis still in liquid condition and While continuing mixing, there is addedabout 8 to 20% of hexamethylenetetramine by weight of the resin and thetwo permitted to partially react and then cooled, or permitted to cool,below the melting point of the modified resin, and to arrest furtherreaction, while continuing mixing and cooling to reduce the mix todiscrete particle form.

As distinguished from the processes described in the aforementionedcopending applications, in accordance with the present invention we havenow found that substantially free-flowing and storage-stable discreteparticles of sand individually coated with a layer of potentiallythermosetting resin composition, may be formed by a cold process, i.e.,without the aid of heat such as the employment of preheated sand orintroduction of heat otherwise to the mixing vessel, by combining withor building into the resin-coated sand an unctuous material, the entireprocess being conducted at normal or prevailing conditions ofatmospheric temperature, and thus at great economy. In the process ofthe present invention the unctuous material not only provides alubricant in the mixing process to facilitate the mixing and coatingwith reduction in power requirement and wear of equipment and a partingcompound to facilitate removal of the formed mold or shell from themetallic pattern in the aforementioned process for production of shellmolds, but additionally provides a marked improvement in the tensilestrength of products formed from the coated sand, such as shell molds orcores, and further substantially eliminates ta'ckiness of the resincoated sand which might result when-using normally liquid resins in theabsence of the unctuous material.

In accordance with the present invention we prefer to use a normallyliquid novolak, i.e., an acid-catalyzed reaction product of from about.5 to about .73 mol of formaldehyde per mol of phenol formed as a clear,oily liquid resin layer at the end of the reaction and dchydrated to asolids content of from about 70% to about by weight, and if desired thesolids content may be further increased by removing volatiles. However,We find the liquid resin suitable at the indicated solids content,although it may be advantageous to add about 10% of ethanol by Weight ofthe resin to reduce tack, particularly when it is employed at a reducedvolatile, i.e., increased solids content.

This is distinguished from the employment of solid novolaks, i.e., thoseformed with from about .75 to about .90 mol of formaldehyde per mol ofphenol and are solid at room temperature. Such solid resins, althoughuseful in the present process, are not fully equivalent in that theyrequire that the resin be dissolved in at least equal parts of alcoholwhich is uneconomical, particularly since the present process operatedwithout the aid of heat renders recovery unfeasible. Although theemployment of such normally solid resins in solution results in endproducts of lower tensile strength than when employing the normallyliquid resins, the tensile strength is considerably enhanced by theunctuous material in each case, and in each instance the unctuousmaterial tends to eliminate tack and makes processing of the mixeseasier.

In accordance with the practice of the preferred embodiment of thepresent invention, sand to be coated is placed at room temperature intoa suitable intensive mixing device, such as a Simpson muller,Werner-Pfleider mixer, Beardsley-Piper mixer, or the like, and thenormally liquid novolak resin is added thereto while mixing. The amountof resin added is to some extent related to the character of the sand tobe coated. Thus with a washed and dried AFS silica sand such as Wedron60M,

from about 0.5% to about 2.5% of resin solids per weight of sand isadequate as compared with the requirement of at least 6% or more whenpreviously employing dry powdered resin-sand mixes. With other sandswhich include some absorptive clay content somewhat larger amounts, forexample up to 45% of the resin, may be required. Hexamethylenetetraminefor eventual conversion of the novolak resin to the thermoset orinfusible condition is added to the mix, preferably in powdered form, inthe proportion of from about 8 to about 20% by weight of the resinsolids at any desired point, either initially with the sand, with theresin, or on subsequent addition of the unctuous additive. Due to theabsence of heat in the process, there does not appear to be any rapidreaction between the resin and the hexamethylenetetramine as evidencedby lack of development of a yellow color which results when the two areheated together, accompanied by rapid increase of melt point of theresin on the sand. However, some degree of progressive reaction, even inthe absence of heating occurs, since some change in at least thephysical properties of the resin does result in the present process.Thus if the sand and liquid resin alone are intensively mixed in an18-inch open Simpson laboratory muller, it requires 10 to minutes forthe resin-coated sand to reach a tacky semi-free flowing condition;whereas if the hexamethylenetetramine is added initially, this samecondition is reached in about 5 to 7 minutes. Some increase in meltingpoint of the resin on the sand also takes place.

The unctuous material is preferably added at least in 7 part to the mixafter the resin and hexamethylenetetramine are intimately distributedwith and coated over the sand and when the mix has reached the aboveindicated semi-free flowing condition and mixing continued for severalminutes, at the end of which time the mass is reduced to a free-flowingdiscrete particle condition composed of particles of sand eachindividually coated with a potentially thermosetting resin compositionenveloped by a film of solid, nontacky, unctuous material.

In accordance with presently preferred practice, a portion of theunctuous material is promptly added to the sand with the resin tofacilitate mixing, and the balance then added with further mixing tobreak up clumps and coat the grains with a film of the unctuous materialto provide free-flowing, nontacky, discrete, individually coated sandgrains. In the alternative, all of the wax may be added at one timeeither at the beginning of the mixing operation or at the indicatedintermediate mixing stage.

The unctuous material is added in the amount of from about 2% to about15% by weight of the resin, depending on the sand, type of resin,percent of resin employed, and the nature of the unctuous material, thelarger proportions, particularly when employing relatively softermaterials, providing some clumping tendency desirable in handling thecoated sand for blowing techniques. The harder unctuous materials may beadded in powdered form, and the softer ones added either as pastes orsolutions for the purpose of obtaining good mixing and coat- Suitableunctuous materials are waxes or wax-like materials which are normallysolids, such as microcrystalline and other paraflin waxes, syntheticwaxes such as substituted polyamide of stearic acid described in US.Patent 2,429,664 known as Acrawax C; waxy medium molecular weightpolyethylenes such as Alcowax 7; methylene bis-stearamide known asArmowax; and materials such as calcium, lithium, and zinc stearates.

As previously indicated, the employment of these waxy or unctuousadditives results in an increase of tensile strength of products formedin accordance with the present invention, as compared to products formedby omit ting them, and this holds true regardless of the melting pointof the waxy solid employed, parafiin waxes such as Warco 70 ofapproximately F. melting point and Warco of about 150 F. melting pointbeing equally suitable for increasing tensile strength 100 or morepounds per square inch as waxy materials of higher melting point such asthe described Acrawax C which has a melting point of about 284-289 F.Tensile strengths herein referred to are those determined .-on standardtensile briquets prepared according to the American Foundrymans Societyspecification.

As an example of the process of the present invention, for the purposeof illustration and not limitation, 200 pounds of washed and driedWedron 60 mesh, Ottowa silica sand (SSAFS) was placed into aBeardsly-Piper Speedmuller together with 4 pounds of liquid novolakresin formed from .7 mol of formaldehyde per mol of phenol anddehydrated to a 70% solids content, and .48 pounds of powderedhexamethylenetetramine containing 5% talc. These were mixed at roomtemperature for three minutes with cold air blown through the mixingvessel to facilitate removal of volatiles. At this time the mass startedto break up and .28 pound of Acrawax C was added and mixing continuedfor two more minutes and the product discharged as a free-flowing sandwith some small lumps which were removed by screening. The melting pointof the coating of the resin was F., and test briquets formed by blowingat a pressure of 25 pounds per square inch exhibited a tensile strengthafter cure of 360 pounds per square inch.

As another example, 400 pounds of the same sand was placed in a No. 1 /2open Simpson muller together with .78 pound of powderedhexamethylenetetramine, 1.8 pounds of Acrawax C, and 6 pounds of liquidnovolak resin formed by reacting .725 mol of formaldehyde per mol ofphenol and dehydrated to a solids content of about 72%. These weremulled for seven minutes, at which time the mass started to break up andbecome powdery, and then .30 additional pound of Acrawax C added. Afterfurther mulling for sixteen minutes the mass was discharged in the formof discrete free-flowing coated sand particles. The time cycle of thisprocess can be reduced by providing blowing means to aid in removingvolatiles. The melting point of the coating on the sand was 175 F., andthe standard briquet test showed a tensile strength of 325 pounds persquare inch.

Although we have hereinbefore described and explained our invention withparticular reference to the employment of two-stage resins, i.e. ofnormally liquid novolak resins together with a requisite proportion ofhexamethylenetetramine for placing the resin in a potentiallythermosetting combination or condition, advantageous employment ofone-stage liquid alkali catalyzed phenol-formaldehyde resins may also bemade for minor or full replacement of the liquid two-stage resins. Theseone-stage liquid resins are of thermosetting character and, having beenprepared from at least equi-molar proportions of a phenol and analdehyde, or even an excess of the latter, permit employment of a singlecomponent without addition of an aldehyde to the mix and are moreeconomical to manufacture.

The product resulting from the process of the present invention isparticularly suitable for the production of shell molds and cores bymeans of conventional blowing machines, and if additional green strengthis required to retain the coated sand in the magazine or blowhead ofconventional blowing machines, this may be simply accomplished by addinga small amount, for example about 0.25% of kerosene by weight of thesand.

We claim as our invention:

l. The method of forming a substantially free-flowing mass of nontackysand grains whereof each particle is coated with an adherent layer ofpotentially thermosetting resin, which comprises intimately mixing in amixing vessel sand to be coated together with from about 2% to about 15%of normally solid waxy material by weight of the resin and liquidphenol-aldehyde resin convertible to therrnoset condition, atsubstantially normal atmospheric conditions of temperature, until thesand grains are individually coated with the resin and the waxy materialand blowing air through the mixing vessel while mixing to facilitateremoval of volatiles until the mass is reduced to a substantiallyfree-flowing, nontacky, discrete particle form.

2. The method of forming a substantially free-flowing mass of nontackysand grains whereof each particle is coated with an adherent layer ofpotentially thermosetting resin, which comprises intimately mixing in amixing vessel sand to be coated together with from about 2% to about 15%of normally solid Waxy material by weight of the resin and liquidphenol-aldehyde resin convertible to thermoset condition comprised ofnormally liquid novolak resin and hexamethylenetetramine in amountadequate to ultimately convert the resin to the infusible condition, atsubstantially normal atmospheric conditions of temperature, until thesand grains are individually coated with the resin and the waxy materialand blowing through the mixing vessel while mixing to facilitate removalof volatiles until the mass is reduced to a substantially free-flowing,nontacky, discrete particle form.

3. The method of forming a substantially free-flowing mass of nontackysand grains whereof each particle is coated with an adherent layer ofpotentially thermosetting resin and adapted for the production of shellmolds, which comprises intimately mixing in a mixing vessel sand to becoated together with from about 2% to about 15 of normally solid waxymaterial by weight of the resin, liquid phenol-aldehyde resin having a.solids content of from about to about by weight comprised of normallyliquid novolak resin in an amount to provide from about 0.5% to about 5%of resin solids by Weight of the sand, and hexamethylenetetramineadequate to ultimately convert the resin to the infusible state, atsubstantially normal atmospheric conditions of temperature, until thesand grains are individually coated with the resin and the waxy materialand blowing through the mixing vessel while mixing to facilitate removalof volatiles until the volatile content of said mix has substantiallyvaporized and the mass is reduced to a substantially free-flowing,nontacky, discrete particle form.

4. The process of claim 3, wherein at least a portion of the waxymaterial is added intermediate the mixing after the resin has beendistributed over the sand and the mix is in a tacky semi-free-fiowingcondition.

References Cited in the file of this patent UNITED STATES PATENTS1,508,124 Richardson Sept. 9, 1924 1,631,113 Anderson June 7, 19271,830,489 Sproat Nov. 3, 1931 2,092,903 Brenner Sept. 14, 1937 2,521,614Valyi Sept. 8, 1950 2,706,163 Fitko Apr. 12, 1955

1. THE METHOD OF FORMING A SUBSTANTIALLY FREE-FLOWING MASS OF NONTACKYSAND GRAINS WHEREOF EACH PARTICLE IS COATED WITH AN ADHERENT LAYER OFPOTENTIALLY THERMOSETTING RESIN, WHICH COMPRISES INTIMATELY MIXING IN AMIXING VESSEL SAND TO BE COATED TOGETHER WITH IRON ABOUT 2% TO ABOUT 15%OF NORMALLY SOLID WAXY MATERIAL BY WEIGHT OF THE RESIN AND LIQUIDPHENOL-ALDEHYDE RESIN CONVERTIBLE TO THERMOSET CONDITION, ATSUBSTANTIALLY NORMAL ATMOSPHERIC CONDITIONS OF TEMPERATURE, UNTIL THESAND GRAINS ARE INDIVIDUALLY COATED WITH THE RESIN AND THE WAXYMATEERIAL AND BLOWING AIR THROUGH THE MIXING VESSEL WHILE MIXING TOFACILITATE REMOVAL OF VOLATILE UNTIL THE MASS IS REDUCED TO ASUBSTANTIALLY FREE-FLOWING, NONTACKY, DISCRETE PARTICLE FORM.